This invention relates to a method of forming tubes or pipes from a substantially flat blank of a plastically deformable material.
In forming of tubes or pipes from flat blanks of plastically deformable material such as steel plate using an upper movable die which coacts with a fixed lower die problems are encountered in attaining good roundness of the tube or pipe, as a complete closure of the blank cannot take place since the upper die has to pass through the wall of the tube or pipe. Even if the upper die is made with a slender web section the thickness of this when pressing heavy-gauge plates which require high pressing forces will be so great that the lengthwise slot obtained in the tube or pipe blank will adversely influence the final roundness of the tube or pipe, since in connection with the final pressing together of said slot further deformation will occur of part of the tube or pipe wall previously bent to the desired curvature.
In forming of large cylinders this problem may possibly be solved by utilization of a gap press stand or of a press stand with detachable end sections. This, however, is not possible in the case of long tubes or pipes or in the case of tubes or pipes with a relatively small diameter in relation to the plate thickness. The dimensions of the pressing tools, usually rollers, will then be too great in relation to the diameter of the tube or pipe.
A main object of the invention is to provide a method and apparatus for forming of tubes or pipes by bending of flat blanks, which gives very good roundness to the tubes or pipes even with long lengths and using thick blanks.